The present invention generally relates to fluorescent tubes and more particularly to serpentine fluorescent tubes for use in avionics equipment.
In today's aviation industry, avionics engineers are involved in a continuing quest to improve the optical performance of avionics displays. One particular area of concern is fluorescent lamps for back-lighting liquid crystal displays.
Typically, fluorescent lamps utilized in the avionics industry are serpentine and are constructed by creating a linear transparent glass tube and coating the interior of the tube with a fluorescent phosphor substance. The linear coated tubes are then fashioned into a serpentine shape by heating the glass tube to its working temperature and then bending the tube.
Another method has been to bend uncoated tubes into a "U" shape and then apply the phosphors via the typical phosphor slurry flush coat method used for linear tubes. Success has been claimed for uniform application of phosphors to "U" shapes using the flush coat method, but "S" shaped or "M" shaped tubes have not been uniformly phosphor coated with the typical slurry deposition method. In order to make "S" or "M" shaped tubes, it has been attempted to weld together 2 or 3 "U" shaped phosphor slurry coated tubes to create "S" and "M" shaped lamps respectively.
While these methods have been used widely in the past all existing methods of fabricating serpentine tubular lamps have several serious draw backs. First of all, when the tubes are bent after coating, the efficiency of the phosphors is diminished as a result of exposure to the high temperature required to allow bending of the tube. Secondly, the bending of the tube results in lacerations or cracks in the phosphor coating. This results in a diminution in luminance uniformity and chromaticity uniformity, as well as the absolute luminance per unit area.
In the method involving welding several slurry coated "U" shaped tubes together, the areas where the "welding" occurs are exposed to high temperatures and the phosphors therein are degraded as a result.
Consequently, there exists a need for improved manufacture of fluorescent tubes for use in the avionics industry, in which phosphor efficiency and uniformity of luminance and chromaticity are not degraded as a result of the fabrication process.